Direct Current (DC) power transmission systems are of interest to use in various situations, for instance when transferring electrical power over long distances.
It is here possible to provide a DC power transmission system as a grid being connected to one or more Alternating Current (AC) systems or AC grids via one or more voltage source converters.
A DC system can here be connected to an AC system in several different ways. It is for instance possible that the DC system is connected via one voltage source converter and a transformer such that there is no grounding on the DC side. It is also possible to connect a DC system to an AC system using separate voltage source converters connected on the DC side between a positive DC potential and ground and a negative DC potential and ground, respectively. In the first case a fault current from the AC side occurs when there are pole-to-pole faults, while in the second case a fault current from the AC side occurs when there are pole-to-ground faults. The first case may also result in fault current at a pole-to-ground fault if the transformer is grounded on both sides or if no transformer is used. One problem with large DC grids is related to selective disconnection of faulty components. If there is for instance a “large” bipolar DC grid, then if a pole-to-ground fault occurs, the voltages on this pole will be severely depressed everywhere in the grid due to the low resistance in the DC grid as compared to the network impedances in a corresponding AC grid. As a consequence essentially no power can be transmitted on this pole as long as the fault is not disconnected. In order to minimize impact on surrounding AC grids and connected end-users, it is important to quickly disconnect the fault. Furthermore, in order to have as much power transfer capability available in the period following the fault, it is important to only disconnect the faulty piece of equipment. These problems will also exist at pole-to-pole faults.
However, it is difficult to break large DC currents, which may be necessary when removing faults in DC power transmission systems. This problem is furthermore getting more severe the more voltage source converters are connected to such a DC power transmission system. There is therefore a need for improvement on this situation.
One way to simplify removal of the fault, which removal may thus involve a breaking of a connection in the DC power transmission system, is through reducing the fault current.
Limiting of the DC current can be performed in a number of ways.
JP 11027853 does for instance describe a rectifier connected to a DC power line including a diode. The rectifier is made up of thyristors. The diode is used to measure the current through the DC power line and the thyristors are controlled to limit this current.
In “A Novel Solid State Fault Current Limiter for DC Power Distribution”, Applied Power Electronics Conference and Exposition, APEC 2008 by Luo, et al a Fault Current Limiter is described. The Fault Current Limiter is provided in series with a DC power line and includes a thyristor in parallel with a transistor, such as an Insulated Gate Bipolar Transistor (IGBT). There is also a capacitor. In operation the thyristor is turned off and the transistor is turned on at the occurrence of a fault. This causes a current running through the transistor and capacitor to gradually increase and the current running through the thyristor to gradually decrease to zero for dependably turning off the thyristor. Once the thyristor has been dependably turned off, the transistor is turned on and off for controlling the fault current.
JP 04138063 describes a bridge type rectifier circuit having thyristors in one leg and switches in another leg.
U.S. Pat. No. 6,624,993 describes a fault current limiting system for direct current circuits. Here a diode is provided in series with a transmission line. If a fault current exceeds a current from a current source that biases the diode open, the diode will cease to conduct and route a fault current through the current source and an inductor.
However, it would be of interest to reduce the fault current using a voltage source converter. A voltage source converter is a converter that is based on switching units, where a switching unit is a combination of a primary switching element with an anti-parallel secondary rectifying element.